Modulation for a data bit stream

ABSTRACT

In some examples, a binary bit stream of input bits is encoded into a three-amplitude bipolar symbol stream of symbols, the encoding using a coding rule that specifies setting a value of a respective symbol of the symbol stream based on a respective input bit of the binary bit stream and a prior input bit of the binary bit stream that is prior to the respective input bit, the coding rule further specifying that adjacent non-zero pulses keep the same polarity. A radio frequency (RF) carrier signal is modulated using the symbol stream to produce a modulated RF signal.

BACKGROUND

Electronic devices such as computers, handheld devices, or other typesof devices can communicate over wired or wireless networks. Wirelessnetworks can include a wireless local area network (WLAN), whichincludes wireless access points (APs) to which devices are able towirelessly connect. Other types of wireless networks include cellularnetworks that include wireless access network nodes to which devices areable to wirelessly connect.

BRIEF DESCRIPTION OF THE DRAWINGS

Some implementations of the present disclosure are described withrespect to the following figures.

FIG. 1 is a graph of a baseband waveform for a regularnon-return-to-zero (NRZ) on-off keying (OOK) signal, according to someexamples.

FIG. 2 a graph of a baseband waveform for a regular pulse-positionmodulation (PPM) signal, according to some examples.

FIG. 3 is a block diagram of an example arrangement that includes atransmitter and a receiver, according to some implementations.

FIG. 4 is a graph of a baseband waveform for a narrowband bipolar on-offkeying (NBB OOK) modulation signal, according to some examples.

FIG. 5 is a schematic diagram of an NBB OOK modulator according to someexamples.

FIG. 6 is a schematic diagram of an NBB OOK modulated signal generatoraccording to some examples.

FIG. 7 illustrates a transition pattern for a switch of the NBB OOKmodulated signal generator according to some examples.

FIG. 8 is a block diagram of a demodulator in a receiver to demodulatean NBB OOK signal according to some examples.

FIG. 9 is a graph of a baseband waveform for a bipolar pulse-positionmodulation (BPPM) signal, according to further examples.

FIG. 10 is a schematic diagram of a BPPM modulated signal generatoraccording to further examples.

FIG. 11 is a block diagram of a demodulator in a receiver to demodulatea BPPM signal according to further examples.

Throughout the drawings, identical reference numbers designate similar,but not necessarily identical, elements. The figures are not necessarilyto scale, and the size of some parts may be exaggerated to more clearlyillustrate the example shown. Moreover, the drawings provide examplesand/or implementations consistent with the description; however, thedescription is not limited to the examples and/or implementationsprovided in the drawings.

DETAILED DESCRIPTION

In the present disclosure, use of the term “a,” “an”, or “the” isintended to include the plural forms as well, unless the context clearlyindicates otherwise. Also, the term “includes,” “including,”“comprises,” “comprising,” “have,” or “having” when used in thisdisclosure specifies the presence of the stated elements, but do notpreclude the presence or addition of other elements.

To communicate data over a network (either a wired network or a wirelessnetwork), modulation is applied. In cases where an electronic devicethat communicates data is a low-power device (such as a battery-powereddevice or a device powered by another type of power source withrestricted power capacity), a modulation scheme that is used should berelatively simple with low power consumption and latency. Examples ofmodulation schemes include the on-off keying (OOK) modulation scheme orthe pulse-position modulation (PPM) scheme.

As an example, the Institute of Electrical and Electronics Engineers(IEEE) 802.11ba task group is developing specifications for a simplephysical (PHY) and Medium Access Control (MAC) standard for low datarate transmission for the application referred to as wake-up receiver(WUR). In an example scenario, an access point (AP) of a WLAN transmitsa WUR signal to wake up a battery-powered device in deep sleep beforestarting to send the payload data. In general, a WUR receiver, which ispowered by a small battery or button cell or other power source that hasa low power capacity, has to be simple with very low power consumptionand latency. However, a WUR transmitter in the AP can be more complexand can have a higher power consumption than a WUR receiver, since theAP has a more robust power source.

Although reference is made to IEEE 802.11 standards in some examples, itis noted that in other examples, modulation solutions according to someimplementations of the present disclosure can be applied to other typesof communications networks, such as Long-Term Evolution (LTE) cellularnetworks operating according to standards provided by the ThirdGeneration Partnership Project (3GPP), 5^(th) Generation (5G) and beyondcellular networks, other types of wireless networks, and/or wirednetworks.

A modulator in a transmitter that applies a modulation scheme receives astream of input data bits that is to be transmitted. The input data bitsare mapped to a stream of symbols, where each symbol represents arespective group of one or more data bits of the input data bit stream,and the symbols collectively form a sequence that is used to modulateone or more radio frequency (RF) carriers that are used to communicatedata over a communication medium (either a wireless medium or a wiredmedium).

An OOK signal (a signal modulated using the OOK modulation scheme) thatis transmitted from a transmitter to a receiver can be non-coherentlydemodulated in the receiver with a very simple envelope detector (ED).For example, an envelope detector used by a receiver for demodulating anOOK signal includes the following passive components: a diode, acapacitor, and a resistor. With such a simple demodulationimplementation, the power consumption of the receiver is very low. As aresult, the OOK modulation scheme can be a viable modulation scheme foruse with low-power receivers.

The on-off key (OOK) modulation is a simple binary modulation scheme. AnOOK signal can be described by its baseband signal, s(t),

$\begin{matrix}{{{s(t)} = {\Sigma_{k}a_{k}{g\left( \frac{t - {kT}}{T} \right)}}},} & \left( {{Eq}.\mspace{14mu} 1} \right)\end{matrix}$where T is the interval of each symbol, a_(k) represents an input binarybit (which can have a value of 0 or 1), and

$g\left( \frac{t - {kT}}{T} \right)$represents a pulse waveform in the duration from kT to (k+1)T.

A non-return-to-zero (NRZ) OOK signal is an OOK signal where the pulsewaveform,

${g\left( \frac{t - {kT}}{T} \right)},$is based on an NRZ gate function, as set forth below:

$\begin{matrix}{{g(t)} = \left\{ {\begin{matrix}{1,} & {{{for}\mspace{14mu} 0} \leq t < 1} \\{0,} & {otherwise}\end{matrix}.} \right.} & \left( {{Eq}.\mspace{14mu} 2} \right)\end{matrix}$

According to Eq. 2, the pulse waveform g(t) has a value of 1 in the timewindow of 0 to T, and a value of zero outside this time window. Eq. 1and Eq. 2 indicate that when an input data bit a_(k) is 1, an RF pulseof unit amplitude and duration T is transmitted, and, when an input databit a_(k) is 0, no signal is transmitted. FIG. 1 shows an example of thebaseband OOK waveform 102 corresponding to an input data bit stream,{a_(k)}={1, 1, 0, 1, 0, 1, 0, 0, 1, 1, 1, 0, 1}.

In the pulse-position modulation (PPM) scheme, which is another binarymodulation scheme, each symbol interval of T is split into two equalsubintervals, wherein one of the two subintervals is used to transmit apositive NRZ RF pulse of duration T/2 and another subinterval is idle.Corresponding to an input bit a_(k), the positive NRZ pulse istransmitted in the first or the second subinterval, depending on thevalue of a_(k). In other words, the information of the input bit iscarried by the pulse position. FIG. 2 shows an example of the basebandwaveform 202 of the PPM signal assuming that a_(k)=0 and a_(k)=1 arerespectively represented by the first and second subintervals where theNRZ pulse is transmitted.

With the PPM scheme, similar to the Manchester code used for binary datacommunication, a PPM signal (a signal modulated using the PPM scheme)transmitted from a transmitter to a receiver has a feature that there isan amplitude level transition in every symbol interval. This helps thereceiver to extract clock timing information from the received PPMsignal to achieve clock synchronization with a clock of the transmitter,even in the presence of long strings of 0 s or 1 s in the input bitstream.

The passband null-to-null bandwidth of an NRZ OOK signal is 2/T. Thismeans that approximately a bandwidth of 2/T Hertz (Hz) is needed totransmit a symbol in T seconds. Furthermore, if the PPM scheme insteadof the OOK modulation scheme is used to facilitate clock synchronizationin the receiver, an even wider bandwidth (as wide as 4/T) is needed.These bandwidth requirements correspond to lower bandwidth efficiencies,since from the theory of Nyquist rate, the minimum passband bandwidthfor transmitting a symbol in T seconds without inter-symbol interferenceis 1/T Hz. In communication systems, frequency bandwidth is a preciousresource, and a higher bandwidth efficiency of a transmitted signal froma transmitter to a receiver is desired.

At the same time, an important constraint specific to the application ofa low-power device is that the modulated signal should be detectable inthe low-power receiver with the same simple envelope detector (ED) usedfor demodulating the regular NRZ OOK or the regular PPM signal withoutextra complexity, and without degradation of an error rate performance(ERP).

In accordance with some implementations of the present disclosure,modified versions of the OOK modulation scheme and/or the PPM scheme areprovided. The modified versions of the OOK and PPM schemes allow fordemodulation using similar receiver complexity as for demodulatingregular OOK or PPM signals, respectively, while achieving higherspectrum/bandwidth efficiencies.

In some examples, a modified version of the OOK modulation scheme isreferred to as a narrowband bipolar OOK (NBB OOK), while a modifiedversion of the PPM scheme is referred to as a bipolar pulse-positionmodulation (BPPM) scheme. In some examples, while the signals of thesemodified modulation schemes can be non-coherently demodulated using thesame demodulation schemes for the regular OOK and regular PPM,respectively, with the similar receiver complexity and withoutdegradation of bit error rate (BER) performance, they have the advantageof providing higher spectrum efficiencies.

FIG. 3 is a block diagram of an example arrangement including atransmitter 300 and a receiver 302 that are able to use modulationschemes according to some implementations of the present disclosure. Ina WLAN, the transmitter 300 can be an AP, while the receiver 302 can bean electronic device, such as a low-power device. Examples of electronicdevices can include sensors, cameras, thermostats, household appliances,vehicles, and so forth. In further examples, electronic devices caninclude smartphones, wearable devices, tablet computers, notebookcomputers, desktop computers, server computers, communication nodes(e.g., switches or routers), storage devices, game appliances,television set-top boxes, and so forth.

In some examples, receivers can include Internet-of-Things (IoT)devices. In addition to network traffic communicated by computers,smartphones, wearable devices, and the like, the IoT technology paradigmcontemplates that other types of devices, including sensors, cameras,thermostats, household appliances, vehicles, have network connectivityto allow the devices to communicate respective data.

The transmitter 300 includes a modulator 302 that has a modulated signalgenerator 304 according to some examples. The modulated signal generator304 receives input binary bits of a data bit stream 306 (also referredto as a binary bit stream) and a radio frequency (RF) carrier signal308, and produces an output modulated signal 310 using modulationschemes according to some implementations of the present disclosure. Theoutput modulated signal 310 is produced by modulating the RF carriersignal 308 using symbols produced by the modulation scheme according tosome implementations of the present disclosure.

The data bit stream 306 can be received from another processing circuitin the transmitter 300, such as a processor, an input/output (I/O)electronic component, and so forth.

The output modulated signal 310 is provided to a transmission circuit312. In some examples, the transmission circuit 312 can include an RFpower amplifier that amplifies the output modulated signal 310 fortransmission by an antenna 314 of the transmitter 300 over the air as atransmitted signal 316.

The transmitted signal 316 is received by an antenna 320 of the receiver302. The receiver 302 includes a demodulator 322 that includes anenvelope detector 324 and a comparator 326. The envelope detector 322 isan electronic circuit that receives a signal of a specified frequency(the frequency of the RF carrier signal 308) and produces an output thatis the envelope of the received RF signal.

The output of the envelope detector 324 is provided to a comparator 326,which compares the output of the envelope detector 324 to a specifiedcriterion (or criteria) to estimate the bit value (e.g., value 0 or 1)of the original data bit value based on the received signal.

The demodulator 322 outputs a data bit stream 328, which corresponds tothe data bit stream 306 transmitted by the transmitter 300 (in otherwords, ideally the sequence of bits in the data bit stream 328 should bethe same as the sequence of bits in the data bit stream 306, unless anerror occurred). The data bit stream 328 including an output binary bitsfrom the demodulator 322 may be sent to another processing circuit inthe receiver 302, such as a processor, an I/O electronic component, andso forth, for further processing

In examples where the receiver 302 is a wake-up receiver (WUR), thetransmitted signal 316 from the transmitter 300 can be a wake-up signalthat is used to awaken the receiver 302 from a power-saving sleep stateto a working state. In other examples, the transmitted signal 116 cancarry other information (control signals or data traffic signals) fromthe transmitter 300 to the receiver 302.

In some examples, for each input bit of the data bit stream 306 receivedby the modulated signal generator 304 in the transmitter 300 produces arespective symbol. Thus, for a sequence of input bits, a correspondingsequence of symbols (referred to as a “symbol stream”) is produced bythe modulated signal generator 304. The modulated signal generator 304encodes the binary bits of the data bit stream 406 into athree-amplitude bipolar symbol stream of symbols. Each symbol of thethree-amplitude bipolar symbol stream of symbols selectively has a valueof −1, 0, or +1.

The symbol stream produced by the modulated signal generator 304 is usedto modulate the RF carrier 308 in the modulated signal generator 304, toproduce the output modulated signal 310.

In accordance with some implementations of the present disclosure, themodulated signal generator 304 applies encoding on the data bit stream306 that uses a coding rule that specifies:

-   -   1) that a value of a current symbol of the symbol stream is set        based on a current input bit of the data bit stream 306 and a        prior input bit of the data bit stream 306 that is immediately        prior to the current input bit,    -   2) that adjacent non-zero pulses of the symbol stream keep the        same polarity, and    -   3) that non-adjacent non-zero pulses of the symbol stream toggle        their polarity alternately (in other words, the non-adjacent        non-zero pulses have opposite polarities).

Note that a symbol can be set to any one of three amplitudes, +1, 0, −1,depending on an input bit value. However, note that a pulse transmittedwithin an interval of the symbol, or a subinterval of the symbol, isnon-zero, i.e., the pulse can be set to a positive polarity (+1) or anegative polarity (−1).

Narrowband Bipolar On-Off Keying (NBB OOK) Modulation Scheme

In some examples, the modulated signal generator 304 of the modulator302 in the transmitter 300 of FIG. 3 is able to generate the outputmodulated signal 310 according to the NBB OOK modulation scheme, whichis a modified version of the regular OOK modulation scheme with greaterspectrum efficiency than the regular OOK modulation scheme.

The data bit stream 306 can be represented as {a_(k)}, k=0, 1, 2, . . ., where a_(k) can have either value 0 or 1 (i.e., either one of the twoelements of a binary set). The modulated signal generator 304 can applyencoding using a coding rule to produce a symbol stream that includessymbols represented as {b_(k)}, k=0, 1, 2, . . . , where b_(k) can alsobe referred to as a baseband signal.

Table 1 below shows an example of a coding rule for mapping an inputbit, a_(k), of the input binary bit stream to a respective value of asymbol, b_(k).

TABLE 1 a_(k) a_(k−1) b_(k) 0 0 0 0 1 0 1 0 Toggle pulse polarity 1 1Keep same pulse polarity

Table 1 includes four rows corresponding to the following combination ofvalues of a_(k) and a_(k-1), denoted as (a_(k), a_(k-1)): first row (0,0); second row (0, 1); third row (1, 0); and fourth row (1, 1). In Table1 above, a_(k-1) represents a prior input bit (received at time instancek−1) that is immediately prior to a_(k) (received at time instance k).Thus, it can be seen that a value of the symbol b_(k) is based on acurrent input bit a_(k) and the immediately prior input bit a_(k-1).

Note that Table 1 shows one example of the coding rule that can beapplied by the NBB OOK modulation scheme. In other examples, a differentcoding rule can be used for the NBB OOK modulation scheme.

According to the coding rule of Table 1, a_(k)=0 (first and second rowsof Table 1) is mapped to b_(k)=0, and a_(k)=1 (third and fourth rows ofTable 1) is mapped to |b_(k)|=1 (i.e., the absolute value of the symbol,b_(k), is 1), that is, a_(k)=1 is either mapped to +1 or mapped to −1.More specifically, according to the coding rule of Table 1, if a_(k)=1and a_(k-1)=0, then the polarity (also referred to as the “pulsepolarity”) of b_(k) is toggled from the polarity of the last non-zerosymbol. The last non-zero symbol is the symbol b_(x) having a non-zerovalue (i.e., +1 or −1) prior to b_(k), where x is the closest prior timeinstance to k. On the other hand, if a_(k)=1 and a_(k-1)=1, then thepulse polarity of the symbol b_(k) is maintained the same as the pulsepolarity of the prior symbol b_(k-1).

The NBB OOK modulation scheme retains the property that |b_(k)|=a_(k).Thus, in a non-coherent receiver, when the envelope detector (e.g., 324in FIG. 3) is making a decision on |b_(k)|, equivalently the envelopedetector is making a decision on a_(k) directly. The benefit of thiscoding rule is that, in the receiver 302, a received signal modulatedusing the NBB OOK modulation scheme can be demodulated using the sameenvelope detector as used for regular NRZ OOK non-coherent demodulationwithout having to provide an extra decoder.

FIG. 4 shows an example of a baseband waveform 402 (a symbol streamincluding symbols b_(k)) for an NBB OOK signal corresponding to an inputdata bit stream 404, {a_(k)}={1, 1, 0, 1, 0, 1, 0, 0, 1, 1, 1, 0, 1}. Inthe example, 13 input bits are present in the input data bit stream 404,at time instances 0, 1, 2, 3, . . . , 12, respectively. The amplitude ofa current symbol, b_(k), is determined by both the current and previousinput bits a_(k) and a_(k-1), as illustrated by Table 1, and asdescribed in more detail further below.

Each symbol b_(k) lasts a specified symbol duration T with an amplitudethat is selectively set to one of −1, 0, +1 based on the current andprevious input bits a_(k) and a_(k-1).

As shown in FIG. 4, at time 1, the value of b₁ is based on thecombination set forth in the fourth row of Table 1, since a₁=1 (attime 1) and a₀=1 (at time 0). According to the fourth row of Table 1, b₁keeps the same polarity as the prior symbol, b₀. Thus, b₁ is set to thesame polarity as b₀.

The value of b₂ at time 2 (b₂ set to 0) is based on the combination setforth in the second row of Table 1, since a₂=0 at time 2, and a₁=1 attime 1. The value of b₃ at time 3 (b₃ set to −1) is based on thecombination in the third row of Table 1, since a₃=1 (at time 3) anda₂=0. (at time 2). According to the third row of Table 1, the value ofb₃ is toggled from the last non-zero symbol, which in the example ofFIG. 4 is b₁ at time 1.

In other examples, instead of the coding rule represented by Table 1, adifferent coding rule can be used for the NBB OOK modulation scheme. Forexample, in the alternative coding rule, Table 1 can be modified suchthat a_(k)=1 is mapped to b_(k)=0, and a_(k)=0 is mapped to |b_(k)|=1.

Thus, the coding rule for NBB OOK modulation scheme can more generallyspecify the following:

-   -   1) If a current input bit a_(k) is equal to a first element of        the binary set (i.e., either 1 or 0), then the current symbol        b_(k) is set to 0. Note that the binary set includes two        elements 0 and 1, where either one can be viewed as the first        element and the other one is the second element.    -   2) If the current input bit a_(k) and the prior input bit        a_(k-1) both are equal to the second element of the binary set,        then the current symbol b_(k) is set to +1 or −1, and the        polarity of the current symbol b_(k) is unchanged from a        polarity of the prior symbol b_(k-1) in the symbol stream.    -   3) If the current input bit a_(k) is equal to the second element        of the binary set, and the prior input bit a_(k-1) is equal to        the first element of the binary set, then the current symbol        b_(k) is +1 or −1, and the polarity of the current symbol b_(k)        is toggled from the polarity of the prior symbol b_(LNZ) which        is the last non-zero symbol prior to b_(k).

Even more generally, the coding rule applied by the NBB OOK modulationscheme specifies that a value of a current symbol of the symbol streamis set based on a current input bit of the data bit stream 404 and aprior input bit of the data bit stream 404 that is immediately prior tothe current input bit, adjacent non-zero pulses of the symbol streamkeep the same polarity, and non-adjacent non-zero pulses of the symbolstream toggle their polarity alternately.

In the case of the NBB OOK modulation scheme, a “pulse” corresponds tothe entire width (T) of a symbol with a non-zero amplitude. As seen inFIG. 4, adjacent non-zero symbols b₀ and b₁ have the same polarity (+1),adjacent non-zero symbols b₈ and b₉ have the same polarity (−1), andadjacent non-zero symbols b₉ and b₁₀ have the same polarity (−1). Alsoas seen in FIG. 4, non-adjacent non-zero pulses b₁ and b₃ of the symbolstream toggle their polarity alternately, non-adjacent non-zero pulsesb₃ and b₅ of the symbol stream toggle their polarity alternately, and soforth.

The baseband NBB OOK waveform varies its amplitude more smoothly thanthe regular NRZ OOK waveform due to the correlation introduced into thesymbols. Thus, the spectrum efficiency of the resultant NBB OOK signalis significantly higher than that of the regular NRZ OOK. Moreover, thebaseband NBB OOK waveform does not have a discrete direct current (DC)component since both +1 and −1 symbol values are used with an equalprobability.

In some examples, the coding rule of Table 1 can be expressed as:b _(k) =a _(k)(a _(k-1) b _(k-1) −ā _(k-1) b _(LNZ))  (Eq. 3)where ā_(k-1)=1−a_(k-1). Eq. 3 describes the following coding rule:

-   -   1) If a_(k)=0, b_(k)=0.    -   2) Otherwise, if a_(k)=a_(k-1)=1, b_(k)=b_(k-1), i.e., transmit        the pulse b_(k) with a polarity that is the same as the prior        symbol.    -   3) Otherwise if a_(k)=1 and a_(k-1)=0, b_(k)=−b_(LNZ), i.e.        transmit the pulse b_(k) with opposite polarity of the last        non-zero symbol (b_(LNZ)).

An important feature of this coding rule is that |b_(k)|=a_(k), that is,a_(k)=0 is mapped to b_(k)=0, and a_(k)=1 is mapped to |b_(k)|=1, asshown in FIG. 4. Thus, in a non-coherent receiver, while the demodulator(e.g., 322 in FIG. 3) is making a decision on |b_(k)|, equivalently thedemodulator is making a decision of a_(k) directly.

FIG. 5 is a block diagram illustrating the relationship between thebaseband and passband signals of the NBB OOK modulation scheme, wherethe coding rule of Eq. 5 is implemented by an encoder 502. The encoder502 receives an input bit a_(k) and outputs a symbol b_(k).

A multiplier 504 multiplies the symbol b_(k) with a pulse waveform 506,

${g\left( \frac{t - {kT}}{T} \right)},$as expressed by Eq. 2 above. The resultant baseband signal 508, s(t),from the multiplier 504 is used to modulate, by a multiplier 512, an RFcarrier signal 510 to produce the NBB OOK RF signal 514. The RF carriersignal 510 is produced by an RF carrier source 511, and is representedas cos(ω_(c)t) where ω_(c)=2πf_(c) with f_(c) representing the RFcarrier frequency.

FIG. 6 is an equivalent modulated signal generator 600 of the NBB OOKsignal 514. The modulated signal generator 600 applies the same logic asthe circuitry shown in FIG. 5. The modulated signal generator 600 is anexample of the modulated signal generator 304 in the transmitter 300 ofFIG. 3. Although a specific example of the modulated signal generator600 is shown in FIG. 6, it is noted that in other examples, othercircuit arrangements can be used to produce the NBB OOK signal 514.

The modulated signal generator 600 includes a switch 602 and amultiplier 604. The switch 602 has three input terminals: +1 terminal, 0terminal, −1 terminal. The +1 terminal is connected to the RF carriersignal 510, the 0 terminal is connected to a 0 value (e.g., ground), andthe −1 terminal is connected to an inverse of the RF carrier signal 510,produced by the multiplier 604 which multiplies the RF carrier signal510 by −1.

Stated differently, the multiplier 604 has a multiplier factor of −1,and the multiplier 604 generates an inverse signal that is the same asthe RF carrier signal 510 but with opposite polarity.

The output of the switch 602 is the NBB OOK RF signal 514. The output ofthe switch 602 is controlled by the value of an input bit a_(k) that isprovided to a control input of the switch 602, such that the output ofthe switch 602 is connected to the 0 terminal in response to the inputbit a_(k) being 0, and the output of the switch 602 stays connected tothe 0 terminal until an input bit a_(k) set to 1 arrives.

In response to the input bit a_(k) being 1, the output of the switch 602is connected to either the +1 terminal or the −1 terminal, according tothe state transition pattern shown in FIG. 7, and the output of theswitch 602 stays connected to the respective +1 or −1 terminal until thenext input bit a_(k) with value 0 arrives. The state transition patternshown in FIG. 7 specifies that when the input bit a_(k) changes from 0to 1, the output of the switch 602 changes from the 0 terminal to the −1terminal if the previous change of the switch 602 was from the +1terminal to the 0 terminal. On the other hand, the state transitionpattern shown in FIG. 7 specifies that when the input bit a_(k) changesfrom 0 to 1, the output of the switch 602 changes from the 0 terminal tothe +1 terminal if the previous change was from the −1 terminal to the 0terminal.

FIG. 8 shows an example of non-coherent demodulator that uses anenvelope detector 802 and a comparator 804 (which correspond to theenvelope detector 324 and the comparator 326, respectively, of FIG. 3).The envelope detector 802 receives an NBB OOK signal r(t) transmittedover the air. The envelope detector 802 outputs a value |r_(k)| thatrepresents the envelope of r(t) for the k-th symbol interval. Thecomparator 804 compares |r_(k)| to a threshold r_(th), and outputs adata bit â_(k) (which represents the estimated (or, recovered) versionof the input data bit a_(k)) that has a value set based on thecomparison performed by the comparator 804. The comparator 804 setsâ_(k)=0 if |r_(k)| is less than the threshold r_(th), and sets â_(k)=1if |r_(k)| is greater than the threshold r_(th).

The envelope detector 802 used for demodulation of the regular NRZ OOKsignal can be directly used for non-coherent demodulation of this NBBOOK signal without an extra decoder, and provides the same error rateperformance in the additive white Gaussian noise (AWGN) channel. Thus,the NBB OOK signal is applicable to a low-power receiver, such as a WUR,without increasing the receiver complexity, although the transmittercomplexity is slightly increased with a more complex generator.

In other examples, the NBB OOK signal produced by the NBB OOK modulatorcan also be demodulated coherently. For coherent demodulation of the NBBOOK signal, the NBB OOK signal can be treated as a three-levelamplitude-shift-keying (ASK) signal on a symbol-by-symbol basis. Inaddition, symbols in the NBB OOK signal produced by a modulator applyingthe NBB OOK modulation scheme are correlated. As a result, coherentdemodulation can be carried out jointly on multiple symbols to takeadvantage of that correlation, rather than on each of the symbolsindividually (i.e., on a symbol-by-symbol basis). For example, a Viterbialgorithm can be applied to demodulate an NBB OOK signal, which canfurther improve the error rate performance. This is an additionalbenefit of the NBB OOK compared with the regular NRZ OOK in someexamples, since the regular NRZ OOK can be demodulated only on asymbol-by-symbol basis.

Bipolar Pulse-Position Modulation (BPPM) Scheme

With the regular PPM modulation scheme, each symbol interval of T issplit into two equal subintervals, each subinterval being of durationT/2, wherein one of the two subintervals is set to idle (i.e., notransmission). Corresponding to an input bit a_(k), a positive NRZ pulseof duration T/2 is transmitted in the first or the second subinterval,depending on the value of a_(k). In other words, the information of theinput bit is carried by the pulse position. Without loss of thegenerality, it is assumed that a_(k)=0 is represented by transmittingthe pulse in the first subinterval, and a_(k)=1 is represented bytransmitting the pulse in the second subinterval. FIG. 2 shows thewaveform of the baseband PPM signal under the foregoing assumption. Itis noted that the opposite assumption is also possible.

In alternative examples, the opposite assumption can be used, wherea_(k)=0 is represented by transmitting the pulse in the secondsubinterval, and a_(k)=1 is represented by transmitting the pulse in thefirst subinterval. With the regular PPM scheme, each symbol has a valueof zero in one subinterval and a NRZ pulse of positive polarity inanother subinterval.

In accordance with some implementations of the present disclosure, theBPPM scheme can provide improved spectrum efficiency with respect to theregular PPM scheme by introducing correlation into the symbol stream. Inthe BPPM scheme, the BPPM signal is a three-level bipolar signal, thatis, the signal amplitude is +1, 0 or −1, with correlation introducedinto the symbols. With the BPPM scheme, a pulse of duration of T/2 istransmitted in the first or the second subinterval to represent thevalue of an input bit of a_(k). With the BPPM scheme, each pulse may beof positive or negative polarity, depending on both the current inputbit a_(k) and the previous bit a_(k-1), as illustrated by Table 2 below.This compares with the regular PPM scheme, where all pulses are ofpositive polarity (i.e., no negative polarity).

FIG. 9 shows an example of a baseband waveform 902 for a BPPM signalcorresponding to the same data bit stream 404 {a_(k)}={1, 1, 0, 1, 0, 1,0, 0, 1, 1, 1, 0, 1} as shown in FIG. 4. Within each symbol interval T,there are two subintervals of duration T/2. In some examples, each inputbit a_(k)=0 is represented by transmitting a pulse (with a negative orpositive polarity) in the first subinterval, and each input bit a_(k)=1is represented by transmitting a pulse (with a negative or positivepolarity) in the second subinterval.

Thus, same as with the regular PPM scheme, BPPM provides an amplitudetransition in each BPPM symbol interval. The BPPM signal waveform variesits amplitude more smoothly than the regular PPM waveform, whichprovides an improved spectrum efficiency for the BPPM signal waveform.

Table 2 below shows an example of a coding rule that can be applied todetermine the pulse polarity (+1 or −1) in the respective firstsubinterval (to represent a_(k)=0) or second subinterval (to representa_(k)=1) of a symbol interval.

The pulse polarity of a pulse for a current symbol b_(k) is determinedbased on the value of the current input bit a_(k) and the value of theimmediately prior input bit a_(k-1). The first row of Table 2 specifiesthat for the combination a_(k)=0 and a_(k-1)=0, the pulse polarity inthe first subinterval for the current symbol b_(k) is toggled from thepulse polarity of the previous symbol b_(k-1). The second row of Table 2specifies that for the combination a_(k)=0 and a_(k-1)=1, the pulsepolarity in the first subinterval for the current symbol b_(k) ismaintained the same as the pulse polarity of the previous symbolb_(k-1). The third row of Table 2 specifies that for the combinationa_(k)=1 and a_(k-1)=0, the pulse polarity in the second subinterval forthe current symbol b_(k) is toggled from the pulse polarity of theprevious symbol b_(k-1). The fourth row of Table 2 specifies that forthe combination a_(k)=1 and a_(k-1)=1, the pulse polarity in the secondsubinterval for the current symbol b_(k) is toggled from the pulsepolarity of the previous symbol b_(k-1).

TABLE 2 a_(k) a_(k−1) Pulse Polarity 0 0 Toggle pulse polarity 0 1 Keepsame pulse polarity 1 0 Toggle pulse polarity 1 1 Toggle pulse polarity

More simply, Table 2 indicates that:

-   -   1) if a_(k-1)=1 and a_(k)=0, the polarity of the k-th pulse (for        the current symbol b_(k)) is kept unchanged,    -   2) otherwise, the polarity of the k-th pulse is reversed from        the previous pulse.

Table 2 assumes that each bit a_(k)=0 or a_(k)=1 is represented bytransmitting a pulse in the first or the second subinterval,respectively. Otherwise, if this assumption is reversed such that eachbit a_(k)=1 or a_(k)=0 is represented by transmitting a pulse in thefirst or the second subinterval, respectively, then the second row ofTable 2 is changed to specify “Toggle pulse polarity,” and the third rowof Table 2 is changed to specify “Keep same pulse polarity.”Correspondingly, rule 1) above for Table 2 is changed to “if a_(k-1)=0and a_(k)=1, the polarity of the k-th pulse (for the current symbolb_(k)) is kept unchanged”.

With the BPPM scheme, each symbol of the three-amplitude bipolar symbolstream has a specified time duration T, and the specified time durationT of each symbol is split into two subintervals, each of duration T/2. ANRZ pulse is transmitted in one of the two subintervals of each symboldepending on a value of the current input bit a_(k) of the data bitstream and a value of the prior input bit a_(k-1) of the binary bitstream, where the NRZ pulse transmitted in one of the two subintervalsis selectively set to −1 or +1, and the remaining subinterval is set toidle.

With the BPPM scheme, the coding rule can generally specify that:

-   -   1) if the NRZ pulse of duration T/2 representing the current        input bit a_(k) is transmitted in the first subinterval and the        pulse representing the immediately prior input bit a_(k-1) is        transmitted in the second subinterval, the pulse transmitted in        the first subinterval for the current input bit a_(k) keeps the        same polarity as the pulse transmitted in the second subinterval        for the prior input bit a_(k-1),    -   2) otherwise, the polarity of the pulse representing the current        input bit a_(k) is toggled from the polarity of the pulse        representing the prior input bit a_(k-1).

In a regular PPM signal, all NRZ pulses, each of duration T/2 have thepositive polarity. In contrast, in a BPPM signal, each NRZ pulse haseither a negative or positive polarity, as shown by FIG. 9. The generalrule of the BPPM scheme is that if a previous pulse was transmitted inthe second subinterval and the following pulse is to be transmitted inthe first subinterval, the following pulse should keep the same polarityas the previous pulse (i.e., adjacent non-zero pulses of the symbolstream keep the same polarity); otherwise the following pulse shouldreverse the polarity from the polarity of the previous pulse.

FIG. 10 shows an example of a BPPM modulated signal generator 1000 forproducing a BPPM RF signal 1014. The modulated signal generator 1000 isan example of the modulated signal generator 304 in FIG. 3. Although aspecific example of the modulated signal generator 1000 is shown in FIG.10, it is noted that in other examples, other circuit arrangements canbe used to produce the BPPM RF signal 1014.

The modulated signal generator 1000 includes a bit encoder 1002 and theNBB OOK modulated signal generator 600 of FIG. 6. Whereas the switch 602in FIG. 6 is controlled directly by the value of the current input bita_(k), the switch 602 in FIG. 10 is controlled by c_(l), which is aparameter that is two bits in length produced by encoding a value ofa_(k) into a pair of bits c_(l).

The bit encoder 1002 extends the input bit stream {a_(k)} into a new bitstream of double length, denoted by {c₁}, by mapping a_(k)=0 into a pairof bits (1, 0) and mapping a_(k)=1 into a pair of bits (0, 1). In eachpair of bits in {c₁}, 1 indicates the position of the pulse in the twosubintervals of a symbol. This is consistent with the assumption thata_(k)=0 and a_(k)=1 are represented respectively by the first and secondsubintervals, where the pulse is transmitted. Otherwise if theassumption is reversed, then a_(k)=0 and a_(k)=1 should be mapped into(0, 1) and (1, 0), respectively.

The bit stream {c_(l)} produced by the bit encoder 1002 is provided tothe control input of the switch 602 of the NBB OOK modulated signalgenerator 600, which selectively connects the output BPPM RF signal 1014to one of the +1 terminal, 0 terminal, and −1 terminal based on arespective input bit in c_(l).

In FIG. 10, the output of the switch 602 is controlled by the value ofc_(l), such that the output of the switch 602 is connected to the 0terminal whenever a bit of c_(l) is 0, and stays at the 0 terminal untila new bit of c_(l) of value 1 arrives. On the other hand, when a bit ofc_(l) is 1, the output of the switch 602 is connected to either the +1terminal or the −1 terminal, complying with the state transition patternas shown in FIG. 7, and stays at the respective +1 terminal or −1terminal until a new bit of c_(l) with value 0 arrives. The statetransition pattern shown in FIG. 7 indicates that when a bit of c_(l)changes from 0 to 1, the output of the switch 602 changes from the 0terminal to the −1 terminal if the previous change was from the +1terminal to the 0 terminal, or from 0 terminal to the +1 terminal if theprevious change was from the −1 terminal to the 0 terminal.

In FIG. 10, the NBB OOK modulated signal generator 600 is working at aclock rate of 2/T, since the clock rate of {c_(l)} is twice of the clockrate of {a_(k)}.

The BPPM signal 1014 produced by the BPPM modulated signal generator1000 can be demodulated non-coherently using a non-coherent demodulatoras shown in FIG. 11, which includes an envelope detector 1102 and acomparator 1104 (which correspond to the envelope detector 324 and thecomparator 326, respectively, of FIG. 3). The envelope detector 1102receives an BPPM signal r(t) transmitted over the air. The envelopedetector 1102 outputs a value |r_(l)| that represents the envelope ofr(t) for each subinterval.

The comparator 1104 has two inputs: an x₁ input and an x₂ input. Aswitch 1106 connects the x₁ input to the signal |r_(l)| in response to lbeing even, and connects the x₂ input to the signal |r_(l)| in responseto l being odd. In the example, l=2k or 2k+1, for k=0, 1, . . . .

For each received symbol, envelope detection can be performed by theenvelope detector 1102 in both of the first and second subintervals,yielding two absolute values |r_(2k)| and |r_(2k+1)|, respectively. Thecomparator 1104 compares the value of x₁ (connected to |r_(2k)| by theswitch 1106 in the first subinterval, i.e., l is even) to the value ofx₂ (connected to |r_(2k+1)| by the switch 1106 in the secondsubinterval, i.e., l is odd). The comparator sets â_(k)=1 if x₂>x₁ andâ_(k)=0 otherwise.

The foregoing assumes that a_(k)=0 and a_(k)=1 are respectivelyrepresented by the first and second subintervals where the pulse istransmitted. Otherwise if the assumption is reversed, then the decisionshould be â_(k)=0 if x₂>x₁ and â_(k)=1 otherwise.

In other examples, coherent demodulation of the BPPM signal can beperformed. Compared to the regular PPM signal, the BPPM signal has aspecial feature when coherent demodulation is used. Since symbols in theBPPM signal are correlated, coherent demodulation can be carried outjointly on multiple symbols to take advantage of that correlation,rather than on each of the symbols individually on the symbol-by-symbolbasis. For example, the Viterbi algorithm can be applied to demodulatethe BPPM signal to improve the error rate performance.

System Architecture

The various circuitry described above, such as those in the transmitter300 or receiver 302, including a modulator, a modulated signalgenerator, a demodulator, an envelope detector, and a comparator, can beimplemented using respective hardware processing circuits. A hardwareprocessing circuit can include any one or some combination of amicroprocessor, a digital signal processor, a core of a multi-coremicroprocessor, a microcontroller, a programmable integrated circuit, aprogrammable gate array, or another hardware processing circuit.

In further examples, any or some combination of the foregoing componentsof the transmitter 300 or receiver 302 can be implemented as acombination of a hardware processing circuit and machine-readableinstructions executable on the hardware processing circuit.

The machine-readable instructions can be stored in a non-transitorymachine-readable or computer-readable storage medium. The storage mediumcan include any or some combination of the following: a semiconductormemory device such as a dynamic or static random access memory (a DRAMor SRAM), an erasable and programmable read-only memory (EPROM), anelectrically erasable and programmable read-only memory (EEPROM) andflash memory; a magnetic disk such as a fixed, floppy and removabledisk; another magnetic medium including tape; an optical medium such asa compact disk (CD) or a digital video disk (DVD); or another type ofstorage device. Note that the instructions discussed above can beprovided on one computer-readable or machine-readable storage medium, oralternatively, can be provided on multiple computer-readable ormachine-readable storage media distributed in a large system havingpossibly plural nodes. Such computer-readable or machine-readablestorage medium or media is (are) considered to be part of an article (orarticle of manufacture). An article or article of manufacture can referto any manufactured single component or multiple components. The storagemedium or media can be located either in the machine running themachine-readable instructions, or located at a remote site from whichmachine-readable instructions can be downloaded over a network forexecution.

In the foregoing description, numerous details are set forth to providean understanding of the subject disclosed herein. However,implementations may be practiced without some of these details. Otherimplementations may include modifications and variations from thedetails discussed above. It is intended that the appended claims coversuch modifications and variations.

What is claimed is:
 1. A method of a device comprising a processor,comprising: encoding a binary bit stream of input bits into athree-amplitude bipolar symbol stream of symbols, the encoding using acoding rule that specifies setting a value of a respective symbol of thesymbol stream based on a respective input bit of the binary bit streamand a prior input bit of the binary bit stream that is prior to therespective input bit, the coding rule further specifying that adjacentnon-zero pulses keep the same polarity, wherein a signal of each symbolof the three-amplitude bipolar symbol stream selectively has a value of−1, 0, or +1, wherein each symbol of the three-amplitude bipolar symbolstream has a specified time duration, and the specified time duration ofeach symbol is split into two subintervals, wherein a pulse istransmitted in one of the two subintervals of each symbol depending on avalue of the respective input bit of the binary bit stream and a valueof the prior input bit of the binary bit stream, and wherein the pulsetransmitted in one of the two subintervals is selectively set to −1 or+1, and modulating a radio frequency (RF) carrier signal using thesymbol stream to produce a modulated RF carrier signal.
 2. The method ofclaim 1, wherein the coding rule further specifies that non-adjacentnon-zero pulses of the symbol stream toggle their polarity alternately.3. The method of claim 1, wherein the coding rule further specifies thatif the respective input bit and the prior input bit are each equal to afirst element of a binary set, then a pulse polarity in a firstsubinterval of a current symbol is toggled from a pulse polarity of aprevious symbol.
 4. The method of claim 3, wherein the coding rulefurther specifies that if the respective input bit and the prior inputbit are each equal to a second element of the binary set, then a pulsepolarity in a second subinterval of the current symbol is toggled fromthe pulse polarity of the previous symbol.
 5. The method of claim 4,wherein the coding rule further specifies that if the respective inputbit is equal the second element of the binary set and the prior inputbit is equal the first element of the binary set, then the pulsepolarity in the second subinterval of the current symbol is toggled fromthe pulse polarity of the previous symbol.
 6. The method of claim 5,wherein the coding rule further specifies that if the respective inputbit is equal to the first element of the binary set and the prior inputbit is equal to the second element of the binary set, then the pulsepolarity in the first subinterval of the current symbol is maintainedthe same as the pulse polarity of the previous symbol.
 7. The method ofclaim 1, wherein the coding rule further specifies that: if a pulserepresenting the respective input bit is transmitted in a firstsubinterval and the pulse representing the prior input bit istransmitted in a second subinterval, the pulse transmitted in the firstsubinterval for the respective input bit keeps the same polarity as thepulse transmitted in the second subinterval for the prior input bit,otherwise, the polarity of the pulse representing the respective inputbit is toggled from the polarity of the pulse representing the priorinput bit.
 8. The method of claim 1, further comprising transmitting, bythe device, the modulated RF signal to a receiver to awaken thereceiver.
 9. A transmitter comprising: a modulator to: encode a data bitstream of input bits into a three-amplitude bipolar symbol stream ofsymbols, the encoding using a coding rule that specifies setting a valueof a respective symbol of the symbol stream based on a respective inputbit of the data bit stream and a prior input bit of the data bit streamthat is immediately prior to the respective input bit, the coding rulefurther specifying that adjacent non-zero pulses keep the same polarity,wherein each symbol of the three-amplitude bipolar symbol stream has aspecified time duration, and the specified time duration of each symbolis split into two subintervals, and wherein a pulse is transmitted inone of the two subintervals of each symbol depending on a value of therespective input bit of the data bit stream and a value of the priorinput bit of the data bit stream, wherein the pulse transmitted in oneof the two subintervals is selectively set to −1 or +1; and modulate aradio frequency (RF) carrier signal using the symbol stream to produce amodulated RF carrier signal.
 10. The transmitter of claim 9, wherein thecoding rule further specifies that non-adjacent non-zero pulses of thesymbol stream toggle their polarities alternately.
 11. The transmitterof claim 9, wherein the modulator comprises: a multiplier with amultiplier factor of −1, the multiplier to generate an inversed carriersignal that is the same as a carrier signal but with opposite polarity;and a switch having first, second, and third inputs, a control input,and an output to provide a modulated signal, wherein the first inputreceives the carrier signal, the second input is connected to ground,the third input receives the inversed carrier signal, and the controlinput receives a control signal based on the data bit stream.
 12. Thetransmitter of claim 11, wherein the switch is to connect the output tothe second input in response to a bit of the control signal set to afirst element of a binary set, and the output is to remain connected tothe second input until a bit of the control signal set to a secondelement of the binary set different from the first element of the binaryset is received by the control input of the switch.
 13. The transmitterof claim 12, wherein the output of the switch is connected to one of thefirst input and the third input in response to a bit of the controlsignal set to the second element of the binary set, and the output is toremain connected to the first or third input until a bit of the controlsignal set to the first element of the binary set is received by thecontrol input of the switch.
 14. The transmitter of claim 13, whereinthe output of the switch is connected to the first input in response toa bit of the control signal set to the second element of the binary setif a prior change of the output of the switch is a change in connectionof the output from the third input to the second input, and wherein theoutput of the switch is connected to the third input in response to abit of the control signal set to the second element of the binary set ifa prior change of the output of the switch is a change in connection ofthe output from the first input to the second input.
 15. The transmitterof claim 11, wherein the control signal is the data bit stream.
 16. Thetransmitter of claim 11, further comprising: an encoder to encode thedata bit stream to produce the control signal, wherein the encoderencodes each respective input bit of the data bit stream into arespective pair of bits that form part of the control signal.
 17. Thetransmitter of claim 16, wherein if the respective input bit is set to afirst element of a binary set, then the respective pair of bits includesa first bit set to 0 and a second bit set to 1, and if the respectiveinput bit is set to a second element of the binary set different fromthe first element of the binary set, then the respective pair of bitsincludes the first bit set to 1 and the second bit set to
 0. 18. Thetransmitter of claim 12, wherein a signal of each symbol of thethree-amplitude bipolar symbol stream selectively has a value of −1, 0,or +1.
 19. The transmitter of claim 12, further comprising an antenna totransmit the modulated RF carrier signal.
 20. A receiver comprising: anenvelope detector to detect an envelope of a modulated radio frequency(RF) signal modulated using a modulation scheme that encodes a data bitstream of input bits into a three-amplitude bipolar symbol stream ofsymbols using a coding rule that specifies setting a value of arespective symbol of the symbol stream based on a respective input bitof the binary bit stream and a prior input bit of the binary bit streamthat is prior to the respective input bit, the coding rule furtherspecifying that adjacent non-zero pulses keep the same polarity, whereinthe modulation scheme further splits each symbol of the symbol streaminto two subintervals; and a comparator to compare an output of theenvelope detector to a specified input to produce data bitscorresponding to the input bits, the comparator to compare a value in afirst subinterval of the two subintervals to a value in a secondsubinterval of the two subintervals.
 21. The receiver of claim 20,wherein the specified input comprises a threshold, and the comparatorcompares the output of the envelope detector to the threshold.